Mechanical upstroke jars are well established for use downhole in oil wells and serve to provide an upward jar or pull on sticking tools in the well. Operation of the jar is achieved by the use of a wireline passing down the well from a winch at the surface to a hammer part of the jar, the wireline imparting a considerable momentum force to the hammer part against spring action and which force is impacted on an anvil part of the jar. Such a jar is described in the U.S. Pat. No. 4,607,692 to Zwart, the disclosure of which is incorporated herein by reference. More specifically the jar of U.S. Pat. No. 4,607,692 comprises an outer casing housing the jar hammer part and having an upper end adapted to constitute the anvil part, the casing additionally housing a spring arrangement which includes a spring located on a spindle secured to a lower part of the casing, while a sleeve defining a hammer holder has a flange located under the spring so that the spring is located and acts between a head of the spindle and the sleeve. Connector segments connect the sleeve and the hammer part so enabling the hammer part to be pulled up by the wireline against spring action. A first recess in the casing for receiving the connector segments defines a point of release of the hammer part from the sleeve, from where the hammer part accelerates upwards to impact on the anvil part, while a second lower recess constitutes a trigger location whereat the hammer can be recoupled to the holder via the connector segments in readiness for a further jarring action. To enable different jarring forces to be produced (with wirelines of appropriate diameter present, for example in the range 2.34 mm to 2.74 mm), the jar is made adjustable by virtue of the spindle being screw positioned in the casing thereby enabling different initial compression forces to be applied to the spring. Adjustment of the spindle is effected by the fitting of a socket tool on the spindle at the lower end of the casing. However, there is the disadvantage that this adjustment cannot not be achieved readily as it is necessary to remove the tool fitted to the lower end of the jar casing before access is available enabling the socket tool to be applied to the spindle. Also, when adjusted to provide a higher load, the spring is constantly in compression, and over time the spring may be subject to creep, requiring recalibration of the adjustment mechanism if the release load is to be accurately set by an operator.
It is among the objects of the various aspects of the present invention to obviate or mitigate these disadvantages.